They have probably harvested the cash crop, and these are the dregs. Watermelons will add lots of nutrients back into the soil as they decay over the winter. The seeds might be a problem though. as if they are mature enough to germinate. There could be a bumper crop of voluntary watermelons come up the following year. This would be if they are hybrid melons. Hybrids often will not produce the same type of fruit the second season. or will not produce the same quality of melon.
My grandpa planted a few rows of Black Diamond melons one year. in the 4-acre family garden plot. He sold a lot of 40 and 50 melons. The is following year what came up voluntarily didn't produce anything that even resembled watermelons. but he had a crop of what he called pie melons only fit to be hog food.